This invention relates to polymers useful as the base resin in resist compositions suited for microfabrication. It also relates to resist compositions, especially chemical amplification resist compositions comprising the polymers, and a patterning process using the same.
In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. To the demand for a resist material with a higher resolution and sensitivity, chemical amplification positive working resist materials which are catalyzed by acids generated upon light exposure are effective as disclosed in U.S. Pat. Nos. 4,491,628 and 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Also, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation. Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.30 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.10 micron rule or less, with the trend toward a finer pattern rule being accelerated.
For ArF excimer laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 xcexcm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser (157 nm) which is expected to enable further miniaturization to 0.10 xcexcm or less, more difficulty arises in insuring transparency because it was found that acrylic resins which are used as the base resin for ArF are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) which is used as the base resin for KrF has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level. Since carbonyl groups and carbon-to-carbon double bonds have absorption in proximity to 157 nm as mentioned above, reducing the number of such units is considered to be one effective way for improving transmittance.
It was recently found that introducing fluorine atoms into base polymers makes a great contribution to an improvement in transmittance at 157 nm. It was reported, for example, in Proc. SPIE Vol. 4345 p273 (2001), xe2x80x9cPolymer design for 157 nm chemically amplified resistsxe2x80x9d that in resist compositions comprising a copolymer of tert-butyl xcex1-trifluoromethylacrylate with 5-(2-hydroxy-2,2-bistrifluoromethyl)ethyl-2-norbornene and a copolymer of tert-butyl xcex1-trifluoromethylacrylate with 4-(hydroxy-bistrifluoromethyl)methylstyrene, the absorbance of the polymer at 157 nm is improved to about 3. However, these resins are still insufficient in transparency because it is believed that an absorbance of 2 or less is necessary to form a rectangular pattern at a film thickness of at least 2,000 xc3x85 through F2 exposure.
In this regard, a highly transparent resin having an absorbance of up to 1 is described in Proc. SPIE Vol. 4690 p76 (2002), xe2x80x9cSynthesis of novel fluoropolymers for 157 nm photoresists by cyclo-polymerization.xe2x80x9d This polymer has not only high transparency, but also good substrate adherence. Since alcoholic groups are used as soluble groups, however, this resin has the drawback of a low dissolution rate in over-exposed areas where acid-eliminatable groups have been eliminated.
An object of the invention is to provide a novel polymer having a high transmittance to vacuum ultraviolet radiation of up to 300 nm, especially F2 (157 nm), Kr2 (146 nm), KrAr (134 nm) and Ar2 (126 nm) laser beams, and useful as the base resin in a resist composition. Another object is to provide a resist composition, and especially a chemical amplification resist composition, comprising the polymer, and a patterning process using the same.
It has been found that by substituting carboxylate, in part, for alcoholic hydroxyl groups on the aforementioned highly transparent polymer, the dissolution rate in over-exposed areas of a polymer film can be improved while minimizing a lowering of transparency.
In a first aspect, the present invention provides a polymer comprising recurring units of the following general formulae (1a) and (1b) and having a weight average molecular weight of 1,000 to 500,000. 
Herein R1 is fluorine or a straight, branched or cyclic alkyl or fluorinated alkyl group of 1 to 20 carbon atoms, R2a and R2b each are hydrogen or xe2x80x94R3xe2x80x94CO2R4, and at least one of R2a and R1b contains xe2x80x94R3xe2x80x94CO2R4, wherein R3 is a straight, branched or cyclic alkylene or fluorinated alkylene group of 1 to 10 carbon atoms, and R4 is hydrogen, an acid labile group, an adhesive group or a straight, branched or cyclic alkyl or fluorinated alkyl group of 1 to 20 carbon atoms.
In a second aspect, the present invention provides a resist composition comprising the polymer, and preferably a chemically amplified positive resist composition comprising (A) the polymer, (B) an organic solvent, (C) a photoacid generator, and optionally, (D) a basic compound and/or (E) a dissolution inhibitor.
In a third aspect, the present invention provides a process for forming a resist pattern comprising the steps of applying the resist composition onto a substrate to form a film; heat treating the film and then exposing it to high-energy radiation in a wavelength band of 100 to 180 nm or 1 to 30 nm through a photomask; and optionally heat treating the exposed film and developing it with a developer. The resist film on the substrate preferably has a thickness of at least 0.2 xcexcm. The high-energy radiation is typically a F2 or Ar2 laser beam or soft x-ray.